I bought the bike as second-hand and the seller said that the mineral oil cannot be the problem. He said that the mineral oil in the brakes is one-year-old and his dealer instructed to buy a new bike (so I got mine 2-3 years old, driven 10k km). I cannot really trust the words so the rear brakes (no problem with front brakes) may have discolored oil, a thing to watch for according to Shimanos' guide. The caliper is Shimano BR-M485, and apparently other parts are proper (not verified), original Crescent Sport bike brakes apparently (not verified). I haven't yet checked that the brake pads are over 0.5mm as instructed but I feel it is not the problem, speculating. So possible errors:

not bleed the air from the brake system

discolored oil

not tighted some knob(?)

too worn-out pads

weather: perhaps some water in the tank and it freezes or oil vaporizing

unknown factor (?)

So why would you hydraulic brakes stop working in cold weather? The question is a bit confusing -- the problem started during cold winter and it persists in room temperature. I have earlier repaired inner hubs so is the procedure the same: break thing up to logical pieces and then follow Shimanos insructions or have you found better ways to repair broken hydraulic disk brakes? Again, the term broken is ambigous, the brakes work in a way that you need to press a lot more but now they feel loose (not with the front brake).

Could someone explain what the term bleeding here actually mean?

Must I really use Shimanos' mineral oil and not some cheap substitute?

4 Answers
4

Hydraulics are the same for car brakes, as for these style brakes, as anywhere else in the physical universe.

Liquids have an amusing property that they do not compress well, if at all. (They do of course compress, but not very much in the grand scheme of things, and it takes a lot of pressure to noticeably compress them).

Thus your hand started breaking force is transmitted direct to the brake, through the medium of the fluid.

If the fluid is getting viscous, due to cold weather, it might become thicker, and thus less responsive to your hand pressure.

The reason to bleed air out of the system is that air compresses very nicely, thank you, unlike a liquid, and so destroys the hydraulic principle in use. Bleeding in this context means to release any trapped air inside the hydraulic cylinder.

Same as in car brakes. First the air compresses, absorbing much of the energy being applied to brake, until finally the liquid starts to transmit the energy.

Very insightful answer, did not connect my bike to a car immediately. As weather has got warmer, the slugginess is lost. For the next winter, I must bleed the air but now they work again very well. Thank you.
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user652Feb 6 '11 at 0:08

It could also be a problem of some product having been applied to the disk, for example: lubricant. If you lubed the chain, derailleur, hub or whatever and some of it went to the disk and not cleaned up properly. It can have damaged the brake pads and hence they not braking properly. If that was the case, the solution would be cleaning the disk and polishing or replacing the brake pads.

it happened to me too. i bought a brand new bike from a local store, and the rear brakes had a leakage, so all the rotors where covered in mineral oil.
it doesn't matter how much you clean your brake pads, it wont go very easily.
i sank the pads in water and then boiled for a couple of minutes, i burnt the pads over the oven, and with alcohol. then i sanded the pads with a very thin grit and finally rode the bike braking only with rear's

of course, also cleaned the rotor almost every day for a week with alcohol.

After 3 months, the noise stopped and the pads are getting much sticky and more sensible to the finger, but if you don't want to go through all that, just buy new pads AND CLEAN PERFECTLY THE ROTORS.